1. Field of the Invention
The present invention relates to a dielectric filter and a dielectric duplexer, each having inner conductor formed holes provided inside of a dielectric block, and an outer conductor provided on the outer surface of the block, and a communication apparatus using the same.
2. Description of the Related Art
A conventional dielectric filter using a substantially parallelepiped-shaped dielectric block is formed in which plural inner conductor formed holes having inner conductors on the inner walls thereof are provided, and an outer conductor is provided on the outer surface of the dielectric block. Referring to a dielectric filter of which one end-face functions as an open-face, and the other opposite-end face does as a short-circuiting face, if two adjacent inner conductor formed holes are straight holes having the same inner diameters and straight-line center axes, the resonance frequencies of the even mode and the odd mode between the two resonators composed of the two adjacent inner conductors and the outer conductor become coincident with each other, coupling between the resonators can not be attained.
In order to couple the adjacent two resonators, conventionally, the following methods have been employed.
(1) The resonators are given a step impedance structure by making different the inner diameters on the open-face side and the short-circuiting side of the inner conductor-formed holes, respectively.
(2) The impedances on the open-face side and on the short-circuiting side of the resonators are made different from each other by providing a slit or step for a part of the dielectric block.
(3) An electrode pattern for coupling the resonators is formed on the open-face of the dielectric block.
The above-described conventional dielectric resonators have the following problems to be solved, respectively. In the case of the structure in which inner conductors are formed on the inner walls of inner conductor formed holes, the unloaded Q (Qo) of each resonator is considerably changed with the inner diameter of the inner conductor formed hole. When the ratio of the thickness of a dielectric block to the inner diameter of the inner conductor formed hole is varied, the unloaded Qo has a maximum at a value of the ratio. Whether the ratio is increased or decreased, the Qo is reduced. Therefore, in the case where the inner diameter of the inner conductor formed hole is made different on the open-face side and the short-circuiting-face side, as described in (1), the inner diameter of the inner conductor formed hole can not be optimized in such a manner that the Qo has a maximum, for the whole of the inner conductor formed hole.
When a distorted portion such as a slit, a step, or the like, as described in (2), a concentration area is generated in the current distribution of the inner conductors and the outer conductor, so that the Qo of each resonator is deteriorated.
Further, in the case of the structure in which an electrode pattern is provided on the open-face of an dielectric block, the coupling coefficient as described in (3), the dimensional precision of the electrode printed pattern determines the coupling coefficient between resonators. Accordingly, there arises the problem that a high accuracy is required, and the production is complicated.
To overcome the above described problems, preferred embodiments of the present invention provide a dielectric filter in which no deterioration of the Qo, is caused by deforming the outer shape and size of the dielectric block, the Qo of a resonator is optimized, and the adjustment of coupling can be easily performed.
One preferred embodiment of the present invention provides a dielectric filter, comprising: a dielectric block; a plurality of inner conductor formed holes provided in the dielectric block; inner conductors provided on the inner walls of the inner conductor formed holes; and outer conductor provided on the outer surface of the dielectric block so as to have one opening-face as an open-face of the inner conductor formed holes, and have the other opening-face thereof as a short-circuiting-face; wherein the sectional shape of the inner conductor formed holes are substantially constant in the range from the open-face to the short-circuiting-face, and a step is provided in the intermediate portion of the center axis of at least one inner conductor formed hole.
Another preferred embodiment of the present invention provides a dielectric filter, comprising: a dielectric block; a plurality of inner conductor formed holes provided in the dielectric block; inner conductors provided on the inner walls of the inner conductor formed holes so as to have open-ends on the inner walls of the holes; and outer conductor provided on the outer surface of the dielectric block; wherein the sectional shape of the inner conductor formed holes are substantially constant in the range from one opening-face of the holes to the other opening-face, respectively, and a step is provided in the intermediate portion of the center axis of at least one inner conductor formed hole.
In the above described dielectric filter, each of the inner conductor formed holes may have a square cross section.
According to the above described structure and arrangement, no deterioration of the Qo, caused by deforming the outer-shape of a dielectric block, occurs, the Qo of a resonator can be optimized, and the coupling can be easily adjusted.
Preferably, the ratio d/D is 0.2-0.4 where D represents the width in the short side direction of the dielectric block, and d represents the width of the inner conductor formed hole.
According to the above arrangement, the Qo can be easily optimized by relatively determining the inner diameter of an inner conductor formed hole based on the outer-shape of the dielectric block.
Further, the position of the step may be nearer to one opening-face with respect to the center in the longitudinal direction of the inner conductor formed hole, and the interval between the center axis of the inner conductor formed hole ranging from the step to the other opening-face of the inner conductor formed hole and the center axis of an inner conductor formed hole adjacent to the inner conductor formed hole may be substantially two times the interval between each center axis and the corresponding outer conductor.
According to the above described arrangement, a bias in a current flowing through the outer conductor and the inner conductor can be reduced, and the reduction of the Qo can be inhibited. The ratio of the inner diameter of the inner conductor formed hole based on the outer shape of the dielectric block can be optimized not only in the thickness direction of the dielectric block but also in the direction in which the resonators are arranged, and therefore, the Qo can be further optimized.
Yet another preferred embodiment of the present invention provides a dielectric duplexer comprising the plurality of the dielectric filters described above, the dielectric filters being formed in the single dielectric block.
Yet another preferred embodiment of the present invention provides a communication apparatus including the dielectric filter or the dielectric duplexer described above.
According to the above arrangement, a communication apparatus of which the loss in a high frequency circuit section is small can be formed without the size being enlarged as a whole.
Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings.